Reinforcement form for use in structural composites,ablative structures and the like

ABSTRACT

A REINFORCEMENT FORM WHEREIN MAJOR REINFORCEMENT FIBERS ARE ARRANGED IN A GENERAL SINE WAVE DISPOSITION EXTENDING LONGITUDINALLY AND HAVING OVERLAPPING AMPLITUDES AND UNIFORM PERIODS. WALE FIBERS EXTEND PARALLEL TO THE DIRECTION OF THE MAJOR FIBERS AND TIE FIBERS ARE PROVIDED   TO FORM A TAPE WHICH CAN BE UTILIZED TO CONSTRUCT IMPROVED STRUCTURAL COMPOSITES AND ABLATIVE STRUCTURES.

2, 1971 J. L. LAZAR ETAL 3,565,740 REINFORCEMENT FORM FOR USE IN STRUCTURAL COMPOSITES, ABLATIVE STRUCTURES AND THE LIKE Filed May 6, 1966 2 Sheets-Sheet 1 5 TIE FIBER ,7

REINFORCING v- Q II.---I FIBER WARP DIRECTION /2 wALE FIBER [EYE emigre; da/m A. Aqzar MW/am E IWn/e/"S 23, 1971 J LAZAR ETAL 3,565,740

REINFORCEMENT FORM FOR USE IN STRUCTURAL COMPOSITES. ABLATIVIL STRUCTURES AND THE LIKE Filed May 6, 1966 2 Sheets-Sheet 2 GLASS OR METALLIC M CARBON OR GRAPHITE iys.

United States Patent 3,565,740 REINFORCEMENT FORM FOR USE IN STRUC- TURAL COMPOSITES, ABLATIVE STRUC- TURES AND THE LIKE John L. Lazar and William E. Winters, Euclid, Ohio, as-

signors to TRW Inc., Cleveland, Ohio, a corporation of Ohio Filed May 6, 1966, Ser. No. 548,166 Int. Cl. B32b /12; D04h 1/70, 3/05 US. Cl. 16157 13 Claims ABSTRACT OF THE DISCLOSURE A reinforcement form wherein major reinforcement fibers are arranged in a general sine wave disposition extending longitudinally and having overlapping amplitudes and uniform periods. Wale fibers extend parallel to the direction of the major fibers and tie fibers are provided to form a tape which can be utilized to construct improved structural composites and ablative structures.

On the drawings:

FIG. 1 is a schematic view showing a reinforcement form provided in accordance with the principles of the present invention;

FIG. 2 is a photograph of an actual reinforcement form embodying the sine wave pattern and Wale cords of the present invention.

FIG. 3 is a view illustrating schematically the different yarn materials of which the reinforcing tape is composed; and

FIG. 4 is a perspective view with parts broken away and parts shown in cross section illustrating a stratified molding formed from tape illustrated in FIGS. 1-3.

As shown on the drawings:

The use of reinforcement forms in the construction of ablative sections or structural composites presents a number of problems. Generally, in building up an ablative section, it is desirable to achieve optimized properties of resistance to erosion, insulation, sacrificial ablation and structural stability. It is also desirable to achieve flexibility in designing gradations of those properties through transverse and longitudinal sections, economics and reliability in material transition interfaces.

It is contemplated by the present invention to use a reinforcing form which constitutes a fabric or tape so constructed as to incorporate across its width the various materials desired in a composite structure cross section. The longitudinal fibers in the reinforcement form are arranged in a sine wave form, the wave traveling in the direction of the length of the form if it is in the configuration of a tape. The overlapping rise and fall portions of the wave may be stitched together with fibers which again travel in the longitudinal direction. Different reinforcement materials can be incorporated into one tape. The materials can be isolated from each other but are joined in a common transition Zone so the general effect is one of stripes spaced across the width of the reinforcement form. The adjacent stripes are mingled and joined in a manner which takes into account their mechanical and physical properties, for example, by nesting so that an effective transition zone results.

Ablative structures fabricated with reinforcement forms having the features of the form thus provided and embedded in a suitable resin matrix exhibit unique and desirable properties in respect to thermal insulation and ablation performance because of the fiber geometry. The general fiber orientation can be maintained at a favorable angle in relation with the heated surface of the ablative structure since each fiber is properly held in the matrix 3,555,740 Patented Feb. 23, 1971 but a continuous fiber path from the hot gas side of the ablative structure to the interior or opposite side is not generated.

In ablative composite plastic structures heretofore provided, reinforcement fibers have been used; however, such fibers normally assume the same geometric form as those in the reinforced plastic components. Such forms may be broadly classified as woven cloth, random oriented fiber and collimated and oriented fiber. Adapting such reinforcement forms to ablative applications, without any significant geometric changes, has limited designers and engineers to relatively fixed ablative cross sections. Moreover, the freedom to introduce strata of various materials and fiber orientations better suited to ablation into the design of an ablative structure is limited by bondline matching strength and production problems.

[n accordance with the principles of the present invention, various reinforcing materials may be incorporated in a continuous matrix and the differing materials are effectively intermingled by nesting in the transition sections, thereby eliminating relatively weak bondline interfaces. Such transition sections effectively accommodate properties such as difierences in thermal expansion and modulus and greatly reduce the problems of match- The basic geometric pattern of the reinforcing form contemplated by the present invention is clearly illustrated in the drawings. Major fibers are provided which are shown generally at and constitute either twisted yarn or monofilaments. The major fibers 10 are laid down in a general sine wave pattern with overlapping amplitudes.

In the sample shown in FIGS. 1 and 2 of the drawings, the periods are uniform and congruent.

In order to retain the major reinforcement fibers 10 in the structural configuration of a tape, the overlapping sections are joined longitudinally near the nodes 11 by tie fiber 15 connecting the fiber 10 to the wale fiber or yarn which is disposed parallel to the general direction of the major fiber and identified on the drawings at 12. The construction thus provided permits assembling the reinforcement form into the tape, or similar sheet-like form, for subsequent handling and forming.

The form of the invention is a composite fabric of monolithic design, consisting of currently available high temperature ablative yarns. Note the sinusoidal placement of the longitudinal reinforcing fibers as depicted in FIG. 1. For clarity, the schematic view shows one layer, whereas in use the actual tape or fabric may have multiple layers. Each layer has a similar geometric configuration, with a phase lag relative to the others. As indicated in the schematic view, the construction consists of three distinctly different fibers. The prime fiber 10, of course, is the ablative material. The Wale fiber 12 (longitudinal linear fiber) provides the restraint or control of the sinusoidal placement of the reinforcing fibers. The tie fibers 15 simply tie the ablative yarns into position onto the Wale fiber. An elastomeric yarn may be utilized as the Wale fiber to provide an extensibility feature to the tape, yielding greater dimetral ratio capability than that ohtainable with the use of standard or preconditioned bias tape slit from standard textile weaves.

The geometric configuration of the reinforcement form offers the designer great versatility in tailoring a tape or fabric to a specific application where reinforcing fibers of the appropriate composition are placed in the strategic location and at the proper orientation to achieve a desired performance.

Such versatility is illustrated in the following examples:

(1) Simultaneous wrapping of two or more dissimilar materials, maintaining a stratified but monolithic section;

(2) Design for any desired diametral ratio wrap;

(3) Transversely varying tape thickness for wrapping at continuously varying orientations;

(4) Varying amplitudes and/ or frequencies to provide desired end-grain effects; and

(5) Interspersing dissimilar fibers in a programmed pattern to achieve a graded cross-section effect.

It should be particularly noted that the arrangement provided effects isolation of the yarn to one portion of the width. Thus, there are no continuous fibers going from one edge of the tape to another and the assembly of nearly any combination of major fiber materials may be ac complished to provide a striped pattern. For instance, combinations of TFE, carbon, silica, glass, and metal filaments and yarns can be combined so that these materials will be isolated in strata when molded with a selected plastic matrix, as shown in FIGS. 3 and 4. (As used herein, TFE refers to polytetrafluoroethylene and polymers of that type.)

As is identified in FIG. 3 by appropriate legends, transverse strength across the width is developed by overlapping shear areas at each node where the yarns are closepacked and parallel.

A particularly advantageous feature of the tape reinforcement form provided in accordance with the principles of the present invention, is the possibility of modifying the major fiber wave form so that most of the fiber is perpendicular, or at some optimum angle, to the hot gas interface on the ablative structure. The form of the wave can be modified with a wide range of period to amplitude ratios so as to accentuate certain properties in one direction or the other. Distorting the sine form so that the amplitude is proportionately greater than the period shown in the illustration would accomplish such accentuation. That kind of orientation cannot be accomplished with any woven forms of reinforcing fibers. By using the tape of the present invention in a formed reinforcement pattern, the tape will naturally form more closely packed radial fibers, tending to be normal to an inside surface, when wrapped.

In typical examples, the tape of the present invention can be constructed as follows. A one-inch tape made of silica and carbon can have equal portions of approximately one-half inch each of yarns made of silica and carbon. A transition zone between the materials will be approximately one-eighth inch, depending on the amplitude selected.

With tape so constructed, it is possible to provide a fabric or tape of selected width which is entirely stable for normal handling, i.e., it does, not distort, elongate, curl or bag. Moreover, the tape thus provided is capable of being formed into radii proportions of greater range of outer diameter to inner diameter which are significant improvements over those possible with woven forms of reinforcement.

It is contemplated by the present invention, in referring to the fibers as being disposed in a generally sine wave form, that a minimum of 50% of the yarn length could have a generally transverse edge-to-edge direction, and it can be also arranged to have a minimum of 50% of the yarn length in a generally longitudinal direction. Moreover, loops or doubling back of the yarn in a plane through the thickness is avoided, eliminating degradation of the yarn resulting when compacting pressures are applied perpendicular to the ply.

The wale material may be selected to promote or limit the tape extensibility. Thus, for example, a polypropylene filament may be used in the Wale which will result in a firm construction in which the sine wave period will be maintained in the molding as assembled in the tape.

It is also possible to use, in the wale, a highly expansible and resilient Spandex or Lycra filament, served, or wrapped, with very fine polypropylene filament. Such a combination permits assembly of the material with the desired degree of firmness or extensibility required for fabrication. The percentage of wale material to major reinforcement material is controllable. Generally, it is contemplated by the present invention that the ratio will be in the range of from one to four percent. From a standpoint of providing a satisfactory ablation material, the use of wale material in the range of percentage noted cannot be considered deleterious since the compositions and thermochemical properties approximate those of the usual plastic matrices.

The major reinforcement fibers may be selected from a wide range of materials. Those materials of particular interest in connection with the construction of ablative structures and heatshields include silica, quartz, carbon, graphite, glass, TFE and asbestos. All of such materials are readily available in the general yarn form required for constructing a reinforcement form in the configuration of a tape as herein disclosed.

FIG. 4 shows a stratified molding made of tape provided in accordance with the principles of the present invention and a typical composite laminate with tape reinforcement. The stratified molding is shown generally at 14 and has an inner face 16 and an outer peripheral face 17.

The tape configuration of the present invention is also particularly suited for use in a heatshield. By interweaving fibers of different compositions and molding the tape into the heatshield, bondlines are eliminated and integral structural capability is assured. ()ne particularly useful combination of fibers consists of TFE, graphite and silica. The TFE provides for initial heat dissipation when placed on the surface.

The graphite fibers furnish a strong char during any extreme heat pulses which may exist, while the silica fibers provide the insulation necessary to maintain the structure of integrity and thermal shockresistance.

For some applications, it is desirable to include metal fibers in the composite. Thus, it is contemplated by the present invention that a graphite yarn can be provided which has been prewrapped or coated with a metal filament. The helical-circumferential pattern achieved in a wound tape affords a large number of random contacts with the required surface continuity.

The form of the present invention constitutes a composite fabric of monolithic design consisting of available high temperature ablative yarns. With the fabric of the present invention, it is possible to form a wedge-shaped reinforcement by selectively changing the denier of the yarn, the yarn density or the number of yarn layers. Moreover, grading may be readily accomplished by incorporating varying amounts or types of yarn through the width of the fabric or layers.

While minor modifications might be suggested by those versed in the art, it should be understood that we wish to embody within the scope of the patent warranted hereon all such modifications as reasonably and properly come within the scope of our contribution to the art.

We claim as our invention: 1. A reinforcement form comprising: plural major reinforcement fibers arranged and disposed in a generally sinusoidal form extending longitudinally and each adjoining pair of fibers having overlapping amplitudes to form overlapping portions,

wale fibers extending parallel to the general direction of the major fibers, and

tie fibers interconnecting said overlapping portions of said major fibers to said wale fibers to tie said major fibers in position onto the wale fibers, thereby to retain the fibers in the form of a tape or fabric.

2. A reinforcement form as defined in claim 1 and further characterized by said major fibers having uniform periods in their generally sinusoidal form.

3. A reinforcement form as defined in claim 1 wherein at least 50% of the fibers length extends in a generally transverse edge-wedge direction.

4. A reinforcement form as defined in claim 1 wherein at least 50% of the fibers length extends in a generally longitudinal edge-to-edge direction.

5. A reinforcement form as defined in claim 1 and further characterized by said major fibers comprising a plurality of different ablative yarn materials each arranged in a longitudinally extending row to form a plurality of yarns,

said yarns forming a transition zone between each respective pair of rows, thereby intermingling the ablative materials and eliminating bondlines to optimize properties of the form to resistance to erosion, insulation, sacrificial ablation and structural stability.

6. A reinforcement form as defined in claim 1 wherein said form is graded by varying amounts or types of yarn formed by the major longitudinal fibers through the width of the form.

7. A reinforcement form as defined in claim 1 and further characterized by said major fibers comprising different ablative materials desired in a composite structure cross-section isolated from one another, but joined in a common transition zone, thereby to form stripes of different ablative material spaced across the width of the tape to achieve optimized properties of resistance to erosion, insulation, sacrificial ablation and structural stability.

8. A reinforcement form as defined in claim 7 and further characterized by said major fibers forming an assembly of combined ablative materials selected from the class of reinforcing materials consisting of polytetrafiuoroethylene, carbon, silica, glass, metal filaments and yarns.

9. An ablative structure comprising:

a resin matrix having a hot gas interface and an opposite side,

and a reinforcement form in said matrix comprising fibers extending longitudinally to form yarn arranged in a general sine wave form with overlapping amplitudes and uniform periods, thereby maintaining the general fiber orientation at an angle with respect to said hot gas interface while avoiding continuous fiber paths between said hot gas interface and said opposite side.

10. An ablative structure as defined in claim 9 and further characterized by said yarn having a wave form modified within a range of period to amplitude to dispose most of the fibers perpendicularly with respect to the hot gas interface. 11. An ablative structure as defined in claim 9 and further characterized by at least of the yarn having a generally transverse edge-to-edge direction.

12. A composite heatshield comprising a resin matrix, and a reinforcement tape integrated in said matrix comprising longtiudinally extending yarns disposed in general sine wave form with overlapping amplitudes and uniform periods and constituting yarns of different ablative materials selected to form a layered structure including a first layer comprising polytetrafiuoroethylene fibers for initial heat dissipation, an intermediate layer comprising graphite fibers furnishing a strong char during an extreme heat pulse, and a third layer comprising silica fibers providing insulation necessary for thermal shock resistance, said yarns being nested to eliminate bondline interfaces between different materials. 13. A composite heatshield as defined in claim 12 and further characterized by one of said yarns being prewrapped with a metal filament.

References Cited UNITED STATES PATENTS 2,495,808 1/1950 Colmant 161-60X 2,671,745 3/1954 Slayter 161G1ass Fabric Dig. 2,758,951 8/1956 Case 16160 3,095,338 6/1963 Romanin 161142 3,135,297 6/1964 Nordberg et al 16160X 3,142,960 8/1964 Bluck 35.6 3,189,510 6/1965 Eldred 161143 3,203,849 8/1965 Katz et a1. 161-96 3,301,742 1/1967 Noland et al. 161-170 3,382,123 5/1968 Alexander 156-467 ROBERT F. BURNETT, Primary Examiner W. A. POWELL, Assistant Examiner US. Cl. X.R.

r 156-l81;16l59, 170,166 

